Socket Programming in Java

A socket literally means an electrical device receiving a plug or light bulb to make a connection. And in computer programming it means a method of communication between two programs one acting as the server (aka provider) and the other as the client (aka requester).

Why is Socket Programming required?

Socket programming is used when two machines need to exchange information for example in cases of web browsers, instant messaging applications and peer to peer file sharing systems. One of the machines prepares the socket and sends it across to the other machine for it to return information through this socket.

Socket Programming in Java

Socket Programming needs two processes, the provider and the requester. The following two java files represent the same. You can play with them to understand how simple exchange of messages occurs between the two.

Provider.java

import java.io.*; import java.net.*; public class Provider{       ServerSocket providerSocket;       Socket connection = null;       ObjectOutputStream out;       ObjectInputStream in;       String message;       Provider(){}       void run()       {             try{                   //1. creating a server socket                   providerSocket = new ServerSocket(2004, 10);                   //2. Wait for connection                   System.out.println("Waiting for connection");                   connection = providerSocket.accept();                   System.out.println("Connection received from " + connection.getInetAddress().getHostName());                   //3. get Input and Output streams                   out = new ObjectOutputStream(connection.getOutputStream());                   out.flush();                   in = new ObjectInputStream(connection.getInputStream());                   sendMessage("Connection successful");                   //4. The two parts communicate via the input and output streams                   do{                         try{                               message = (String)in.readObject();                               System.out.println("client>" + message);                               if (message.equals("bye"))                                     sendMessage("bye");                         }                         catch(ClassNotFoundException classnot){                               System.err.println("Data received in unknown format");                         }                   }while(!message.equals("bye"));             }             catch(IOException ioException){                   ioException.printStackTrace();             }             finally{                   //4: Closing connection                   try{                         in.close();                         out.close();                         providerSocket.close();                   }                   catch(IOException ioException){                         ioException.printStackTrace();                   }             }       }       void sendMessage(String msg)       {             try{                   out.writeObject(msg);                   out.flush();                   System.out.println("server>" + msg);             }             catch(IOException ioException){                   ioException.printStackTrace();             }       }       public static void main(String args[])       {             Provider server = new Provider();             while(true){                   server.run();             }       } }

Requester.java

import java.io.*; import java.net.*; public class Requester{       Socket requestSocket;       ObjectOutputStream out;       ObjectInputStream in;       String message;       Requester(){}       void run()       {             try{                   //1. creating a socket to connect to the server                   requestSocket = new Socket("localhost", 2004);                   System.out.println("Connected to localhost in port 2004");                   //2. get Input and Output streams                   out = new ObjectOutputStream(requestSocket.getOutputStream());                                           out.flush();                   in = new ObjectInputStream(requestSocket.getInputStream());                   //3: Communicating with the server                   do{                         try{                               message = (String)in.readObject();                               System.out.println("server>" + message);                               sendMessage("Hi my server");                               message = "bye";                               sendMessage(message);                         }                         catch(ClassNotFoundException classNot){                               System.err.println("data received in unknown format");                         }                   }while(!message.equals("bye"));             }             catch(UnknownHostException unknownHost){                   System.err.println("You are trying to connect to an unknown host!");                                                                   }             catch(IOException ioException){                   ioException.printStackTrace();             }             finally{                   //4: Closing connection                   try{                         in.close();                         out.close();                         requestSocket.close();                   }                   catch(IOException ioException){                         ioException.printStackTrace();                   }             }       }       void sendMessage(String msg)       {             try{                   out.writeObject(msg);                   out.flush();                   System.out.println("client>" + msg);             }             catch(IOException ioException){                   ioException.printStackTrace();             }       }       public static void main(String args[])       {             Requester client = new Requester();             client.run();       } }

You can extend these classes to much more complex use-cases. Hope this helped lay the foundation.